In recent years, as a mechano-electrical transducer for application to a driving element, a sensor, or the like, a piezoelectric body such as Pb(Zr, Ti)O3 has been used. Such a piezoelectric body in the form of a thin film formed over a substrate of Si or the like is expected to be applied to MEMS (micro-electro-mechanical systems) elements.
In producing a MEMS element, high-precision processing using a semiconductor process technology such as photolithography can be used, thus allowing the element to be reduced in size and to have an increased packing density. Particularly by densely and collectively fabricating elements on a relatively large Si wafer such as of 6 inches or 8 inches in diameter, compared with a case of single wafer production in which elements are produced one by one, significant cost reduction can be achieved.
Furthermore, with a piezoelectric body used in the form of a thin film and a device formed in a MEMS configuration, mechano-electrical transduction efficiency is improved, and this has led further to creation of new added values such as improvements in sensitivity and characteristics of the device. For example, in a case of a thermal sensor, having the MEMS configuration, the thermal sensor is reduced in thermal conductance, so that a measurement sensitivity thereof can be increased, and in a case of an ink-jet head for a printer, nozzles thereof are provided at an increased packing density, so that high-definition patterning can be performed.
As a material of a piezoelectric body thin film (piezoelectric thin film), a crystal constituted of Pb, Zr, Ti, and O, which is referred to as PZT (lead zirconate titanate), is often used. PZT exhibits an excellent piezoelectric effect when having an ABO3-type perovskite structure shown in FIG. 16 and thus should be used in a perovskite single-phase form. The shape of a unit cell of a crystal of PZT having the perovskite structure varies depending on a ratio between Ti and Zr, which are atoms to be in a B site. That is, when the content of Ti is large, PZT has a tetragonal crystal lattice, and when the content of Zr is large, PZT has a rhombohedral crystal lattice. When a molar ratio between Zr and Ti is in the vicinity of 52:48, both of these crystal structures are present, and a phase boundary at which this composition ratio is achieved is referred to as an MPB (morphotoropic phase boundary). With such an MPB composition, piezoelectric characteristics such as a piezoelectric constant, a polarization value, and a dielectric constant are enhanced to the maximum, and thus a piezoelectric thin film having the MPB composition has been vigorously used.
When a piezoelectric thin film is used as a MEMS driving element, in order to meet a required level of displacement generating force, the piezoelectric thin, film needs to be formed in a thickness of 3 μm to 5 μm. As a method for forming the piezoelectric thin film over a substrate of Si or the like, there are known a chemical film formation method such as CVD, a physical method such as sputtering or ion plating, and a liquid phase growth method such as a sol-gel method, and it is important that conditions for obtaining a perovskite single-phase film are found so as to be suited to an adopted one of the film formation methods.
By the way, recent years have seen a demand for a MEMS driving element having a further increased packing density and a higher output. In order to realize such a driving element, there is demanded a piezoelectric thin film that, achieves a value (absolute value) of a piezoelectric constant d31, which is an index to evaluate piezoelectric characteristics, of not less than 180 [pm/V].
In order, therefore, to realize a piezoelectric thin film having high piezoelectric characteristics, it has been studied to extend measures to enhance piezoelectric characteristics used for bulk ceramic to thin films. One of such measures to enhance piezoelectric characteristics is to control a polarization domain so as to utilize non-180° domain rotation.
A perovskite crystal of PZT or the like is a ferroelectric body having spontaneous polarization Ps when no voltage is applied thereto. A region in which a direction of the spontaneous polarization Ps is aligned is referred to herein as a polarization domain. A possible direction of the spontaneous polarization Ps varies depending on the shape of a unit cell of a crystal, and is a <100> axis direction when the unit cell is tetragonal and a <111> axis direction when the unit cell is rhombohedral. It is assumed that the <100> axis direction collectively refers to six equivalent directions in total, which are [100], [010], and [001] directions and directions opposite thereto. Furthermore, it is assumed that the <111> axis direction collectively refers to eight equivalent directions in total, which are [111], [−111], [1−11], and [−1−11] directions and directions opposite thereto. Incidentally, FIG. 17A shows a direction of spontaneous polarization Ps of a tetragonal crystal, and FIG. 17B shows a direction of spontaneous polarization Ps of a rhombohedral crystal.
As a non-180° domain, in a case of a tetragonal crystal, there exists, for example, a domain having polarization in the [100] direction inclined in terms of a polarization direction at an angle of 90° with respect to a domain having polarization in the [001] direction. Furthermore, in a case of a rhombohedral crystal, there exists, for example, a domain having polarization in the [−1−11] direction inclined in terms of a polarization direction at an angle of 71° with respect to a domain having polarization in the [111] direction. Among such domains, one having a maximum displacement amount is a 90°-rotated domain of the tetragonal crystal, and various studies for efficiently utilizing 90° domain rotation of the tetragonal crystal have been conducted. The following describes in detail the 90° rotation of the tetragonal crystal.
FIG. 18 shows a piezoelectric distortion ΔX1 that commonly occurs in a case where, with respect to the domain having polarization in the [001] direction of the tetragonal crystal, an electric field is applied in the [001] direction, and a piezoelectric distortion ΔX2 that occurs in a case where, with respect to the domain having polarization in the [100] direction of the tetragonal crystal, an electric field of the same magnitude is applied in the [001] direction. In this figure, each thick solid arrow indicates a polarization direction. As shown in this figure, the piezoelectric distortion ΔX2 resulting from 90° domain rotation from the [100] direction to the [001] direction is larger than the piezoelectric distortion ΔX1 that commonly occurs. Thus, in a case where such 90° domain rotation of the tetragonal crystal can be caused efficiently every time an electric field is applied, piezoelectric characteristics can be enhanced. From this viewpoint, conceivably, in a tetragonal PZT film, by increasing a region that is a (100)-oriented domain, namely, a domain having polarization in the [100] direction, an effect of domain rotation can be significantly utilized, and thus a higher piezoelectric constant is obtained.
In a film in actual use, however, there often occurs so-called pinning of domains (locking of domains) in which even when the domain in the [100] direction, after being rotated once in a direction of an electric field so that its polarization direction is turned to the [001] direction, is released from application of the electric field, the domain is not turned back into the [100] direction, which is its original polarization direction, so that a crystal is turned into a (001)-oriented crystal. Because of this, when an electric field is applied for the second and following times, there exists a region in which no displacement by domain rotation occurs, resulting in a decrease in displacement.
In order to solve this, for example, in Patent Document 1, based on a finding that pining of domains is caused by a lead atom in a composition, PZP having a lead-deficient film composition is used so as to suppress pinning of domains in an attempt to, in a (100)-oriented tetragonal PZT film, effectively utilize domain rotation, thereby obtaining high piezoelectric characteristics.
Furthermore, energy required to generate a distortion resulting from 90° domain rotation is higher than energy required to generate a piezoelectric distortion that commonly occurs, and thus even if a (100) single-oriented tetragonal PZT film is obtained, a device driving voltage that is commonly used is not sufficient to cause 90° domain rotation therein.
In order to solve this, for example, in Patent Document 2, as a piezoelectric body, a (100) single-oriented film whose orientation axis is inclined from a direction perpendicular to a substrate is formed, and thus non-180° domain rotation is caused reversibly and efficiently (by using a low electric field), so that high piezoelectric characteristics are obtained.